Liquid cooled band edge oscillation prevention for a twt

ABSTRACT

A longitudinally extending liquid cooled attenuator is disposed proximate to and externally of a traveling wave tube coupled cavity slow-wave structure. The attenuator may be a hollow dielectric tube filled with a lossy fluid or a hollow dielectric tube with a lossy solid which is cooled by a moving fluid or a tubular ceramic rod disposed coaxially about an electrically conductive rod. This device lends itself well to the prevention of band-edge oscillations in the coupled cavity traveling wave tube.

United States Patent 1 Winslow Nov. 6, 1973 LIQUID C OOLED BAND EDGE OSCILLATION PREVENTION FOR A TWT [75] Inventor: Lester M. Winslow, Alexandria, Va.

[73] Assignee: The United States of America as represented by the Secretary of the Navy, Washington, DC.

22 Filed: Nov. 22, 1972 21 App]. No.: 308,896

[52] US. Cl 315/35, 313/30, 313/32,

315/39.3 [51] Int. Cl. HOIj 25/34 [58] Field of Search 315/3.5, 3.6; 313/30, 32, 39

[56] References Cited UNITED STATES PATENTS 3,354,347 1l/1967 l-lant 315/35 3,324,338 6/1967 Winslow 315/35 3,329,855 7/1967 Landsbergen 3l5/3.5 3,412,279 11/1968 Allen et a]. 3l5/3.5 3,617,798 11/1971 Marchese 315/35 3,391,299 7/1968 Bodmer et a1. 315/36 Primary Examiner-Rudolph V. Rolinec Assistant Examiner-Saxfield Chatmon, Jr. Attorney-R. S. Sciascia et a].

[57] ABSTRACT A longitudinally extending liquid cooled attenuator is disposed proximate to and externally of a traveling wave tube coupled cavity slow-wave structure. The attenuator may be a hollow dielectric tube filled with a lossy fluid or a hollow dielectric tube with a lossy solid which is cooled by a moving fluid or a tubular ceramic rod disposed coaxially about an electrically conductive rod. This device lends itself well to the prevention of band-edge oscillations in the coupled cavity traveling wave tube.

8 Claims, 5 Drawing Figures LOSS (db) PATENIEDuuv 6191s 3771; 010

SHEET 3EF 3 C) O FIG 5 o IO 0 I I THE RF LOSS AS A FUNCTION OF FREQUENCY I FOR THE coupuzo 8 CAVITY TWT WITH 9 COUPLED um:

| OSCILLATION PREVENTION. 8 I o I o I I J l a l FREQ (GHz) f 050 LIQUID COOLED BAND EDGE OSCILLATION PREVENTION FOR A TWT BACKGROUND tromagnetic wave is propagated along a slow-wave structure, such as a conductive helix wound about the path ofthe electron stream or a folded waveguide type of structure in which a waveguide is effectively wound back and forth across the path of the electrons. The slow-wave structure provides a path of propagation for the electromagnetic wave which is considerably longer than the axial length of the structure, and hence, the

traveling-wave may bemade to effectivelypropagate at nearly the velocity of the electron stream. The interactions between the electrons in the stream and the traveling-wave cause velocity modulations and bunching of the electrons in-the stream. The net result may then be atransfer of energy from the electron beam to the wave traveling along the slow-wave structure.

The present invention is primarily, although not necessarily, concerned with traveling-wave tubes utilizing slow-wave structures of the coupled cavity, or interconnected cell, type. In this type of slow-wave structure a series of interaction cells, or cavities are disposed adjacent to each other sequentially along the axis of the tube. The electron stream passes through each interaction cell, and electromagnetic coupling is provided between each cell and the electron stream. Each interaction cell isalso coupled to an adjacent cell by means of a coupling holeat the end wall defining the cell. Generally, the coupling :holes between adjacent cells are alternately disposed on opposite sides of the axis of the tube, although various other arrangements for staggering the coupling holes are possible and have been employed. When the coupling holes are so arranged, a folded waveguide type of energy propagation results, with the traveling wave energy traversing the length of the tube by entering each interaction cell from one side, crossing the electron stream and then leaving the cell from the other side, thus traveling a sinuous, or serpentine, extended path.

One of the problems encountered in traveling wave tubes of the coupled cavity variety, and especially high power tubes of this type, is a tendency for the tube to oscillate at frequencies near the edges of the tube passband. This problem arises from the fact that for wide band operation the phase velocity of the slow-wave circuit wave and the velocity of the electron beam should be essentially synchronized over as large a range of frequencies as possible; hence, these velocities are also close to synchronism near the upper and lower cutoff frequencies of the tube. Since the interaction impedance is high and the circuit-to-transmission line match is poor at and in the vicinity of the cutoff frequencies, the loop gain for the tube, or even for a section of the tube, may be sufficiently large for oscillations to start.

One technique which has been used to solve this oscillation problem involves coupling to the slow-wave structure interaction cells specially designed cavities of the cavity. This technique is able to attenuate energy at those frequencies where the tube is most likely to OS- cillate without substantially affecting energy at frequencies throughout the remainder of the tube passband. Furthermore a minimum reflection coefficient is capable of being provided as disclosed in U. S. Pat. No. 3,324,338 since a low reflection coefficient is desirable in preventing large fluctuations in gain as a function of frequency at the low end of the tube passband. Furthermore it can be shown that at the lowest and highest frequencies in the passband the derivation of the frequency as a function of the phase shift is zero. At these points the group velocity of the circuit is zero and it becomes possible for the tube to oscillate in a deleterious manner.

Therefore I have developed a device to prevent such oscillations from occurring without the extraction of a substantial amount of power so the beam does not slow down to a point which permits synchronism with the upper cutoff frequency and therefore cause drive induced oscillations.

OBJECTS It is an object of this invention to provide an rf loss at the band edge which is 4 db/cavity so that most band edge oscillations maybe prevented.

Another object of this invention is to further increase the thermal capacity of the circuit by absorbing power directly by a moving fluid.

A further object is to provide a. traveling wave tube in which any tendency for the tube to oscillate in the vicinity of the edges is substantially reduced by employing a hollow dielectric tube filled with a lossy fluid.

Other objects, advantages and'novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings wherein:

THE DRAWINGS FIG. 1 is an overall view, partly in longitudinal section and partly broken away, of a. traveling-wave tube constructed in accordance with the invention;

FIG. 2 is a cross-sectional view taken along the line 2-2;

FIG. 3 is a cross-section similar to FIG. 2 except that the slow-wave structure employs a star shaped crosssection hollow dielectric tube;

FIG. 4 depicts the use of a circullar cross-section hollow dielectric tube in the slow-wave structure; and

FIG. 5 is a graph of the rf loss as a function of frequency for the coupled wave TWT having the coupled line oscillation prevention in accordance with this invention.

SUMMARY The traveling wave tube of the present invention includes means for providing a streamof electrons along apredetermined path and a slow-wave structure having a plurality of intercoupled interaction cavities disposed sequentially along and about the electron stream path for propagating electromagnetic wave energy in such manner that it interacts with the stream of electrons. An attenuating transmission line including a hollow dielectric tube (of various cross-sections) with a lossy fluid, or a hollow dielectric tube with a lossy solid which is cooled by moving fluid is disposed proximate to and externally of the slowwave structure cavities with the longitudinal axis of the tube being parallel with the electron stream path so as to reduce bandedge oscillations.

DETAILED DESCRIPTION Referring now to the drawings, and more particularly to FIG. 1, the reference numeral designates generally a traveling-wave tube.

Coupled to the input end of the arrangement 12 is an input waveguide transducer 14 which includes an impedance step transformer 16. A flange 18 is provided for coupling the assembled traveling-wave tube 10 to an external waveguide or any other microwave transmission line. The construction of the flange 18 may include a microwave window (not shown) transparent to microwave energy but capable of maintaining a vacuum within the traveling-wave tube 10. At the output end of the arrangement 12 an output transducer 20 is provided which is substantially similar to the input transducer 14 and which includes an impedance step transformer 22 and a coupling flange 24, which elements are similar to the elements 16 and 18, respectively, of the input transducer 14. For vacuum pumping or out-gassing the traveling-wave tube 10 during manufacture, a double-ended pumping tube 26 is connected to both of the input and output waveguide transducers l4 and 20.

An electron' gun 28 is disposed at one end of the traveling-wave tube 10 which, although illustrated as the input end in FIG. 1, may alternatively be the output end if a backward wave device is desired. The electron gun 28 functions to project a stream of electrons along the axis of the tube 10 and may be of any conventional construction well known in the art.

At the output-of the traveling-wave tube 10 there is provided a cooled collector structure 30 for collecting the electrons in the stream. The collector is conventional and may be of any form well known in the art. The construction of a slow-wave structure and magnetic focusing system for the traveling-wave tube 10 are illustrated in more detail in FIG. 2. Thus, a plurality of essentially annular disk-shaped focusing magnets 32 are interposed between a plurality of ferromagnetic pole pieces 34. Asillustrated in FIG. 2, the magnets 32 may be diametrically split into two sections 32a and 32b for convenience during assembly of the tube. The ferromagnetic pole pieces 34 extend radially inwardly of the magnets 32 to approximately the perimeter of the region adapted to contain the axial electron stream. The individual pole pieces are constructed in such a manner that a short drift tube, or ferrule, 36 is provided at the inner extremity of each pole piece. The drift tube 36 is in the form ofa cylindrical extension, or lip, protruding axially along the path of the electron stream from both surfaces of pole piece 34. The drift tubes 36 are provided with central and axially aligned apertures 38 to provide a passage for the flow of the electron beam. Adjacent ones of the drift tubes 36 are separated by a gap (not shown) which functions as a magnetic gap to prvide a focusing lens for the electron beam and Disposed radially within each of the magnets 32 is a slow-wave circuit spacer element 42 of a conductive non-magnetic material such as copper. Each spacer element 42 has an annular portion of an outer diameter essentially equal to the inner diameter of the magnets 32 and a pair of oppositely disposed ear portions 43 and 44 projecting outwardly from the annular portion.

For interconnecting adjacent interaction cavities an off-center coupling hole 48 is provided through each of the pole pieces 34 to permit the transfer of electromagnetic wave energy from cell to cell. As is illustrated, the coupling holes 48 may be substantially kidney-shaped and may be alternately disposed 180 apart with respect to the drift tubes 36.

It will be apparent that the spacer elements 42 and the portions of the pole pieces 34 projecting inwardly of the spacers 42 not only form an envelope for the tube, but also constitute a slow-wave structure for propagating traveling-wave energy in a serpentine path along the axially traveling electron stream so as to support energy exchange between the electrons of the stream and the traveling-wave.

As has been mentioned above, in prior art travelingwave tubes of the type described there may be a tendency for the tube to oscillate at frequencies near the edges of the slow-wave circuit passband and for the gain to fluctuate excessively at the low frequency end of the passband. The present invention eliminates this tendency by coupling in parallel with the slow-wave circuit at least one lossy transmission line especially designed to introduce relatively wide-band loss, and thereby shape the gain vs. frequency characteristic of the traveling-wave tube.

Specifically referring to FIG. 2, a pair of lossy transmission lines 56 and are longitudinally disposed within auxiliary cavities along the electron beam ferrule 36. As shown in FIG. 2, each transmission line may be of a low loss material such as ceramic or glass. It should be noted that transmission line 60 employs a metal conductor 53, annularly disposed in the center hole 52 while the transmission line 56 depicts a clear center hole 50. Comparing transmission line 56 with line 60 it should be noted that although the proper fluid passing through center hole 50 may sufficiently remove heat and provide the required rf loss, the transmission line 60 is preferable especially for gain flattening. An acceptable fluid includes H O, ethylene glycol, ethyl or methyl alcohol. The metal conductor 53 assures that the loss is wide band and such an arragement is especially desirable for lower band-edge oscillations and gain flattening.

Referring to FIG. 3, it can be seen that star crosssectioned 54, which is ceramic material with a high loss trangent such as Carbolex, is disposed in the center non-conductive tube of the transmission line 56. Heat removing fluid is able to pass through center hole 59 to terials must be secured at both ends of the lossy matealso as an interaction gap in which energy exchange between the electron beam and traveling-wave energy traversing the slow-wave structure occurs.

rial to assure the proper fluid flow. The drawback of one-way flow is that cooling is not as effective as with two-way flow. Cooling is important since a large thermal difference, expansion and contraction are responsible for applying lateral tension and compression on the non-conductive transmission line members. The metal rod 61 disposed in the center provides wide band loss as discussed in the FIG. 2 embodiment. An important advantage of using the ceramic material with a high loss tangent is that low loss oils may be used as a fluid. Such low loss material may include fluorocar-.

bons.

FIG. 4 depicts another arrangement of the structure employed to prevent lead edge oscillations and remove power. Each transmission line 56 and 60 comprise one annular tube inside another. Specifically, transmission line 56 employs a ceramic or glass outer member with a smaller metal tube 57 disposed concentrically inside. High loss fluid flows through hole 59 and returns through annular opening 55. Transmission line 56 in FIG. 4 is identical to transmission line 60 exceptthat metal tube 57 is replaced by a high loss tube 65.

Although FIGS. 2-4 each depict a pair of transmission lines, each of which differ from the other, it should be noted that the particular combinations as shown in the individualfigures should not be construed as a restriction. That is to say, both transmission lines of any cross-section or either may be selected from those disclosed herein.

FIG. 5 is a graph of the RF loss as a function of frequency for the coupled cavity traveling wave tube with the coupled line oscillation prevention as discussed above. Specifically, the RF loss presented by the coupled line is frequency selective as shown in FIG. 4. As a result of this invention, the RF loss at the band edge is much greater than in the operating frequency band of the traveling wave tube. The RF loss at the band edge is much greater than in the operating frequency band of the traveling wave tube. The RF loss/cavity in the band is about 0.4 db while at the nominal oscillation frequency it is greater than 3 db. per cavity. In a standard section of 20 cavities the total band edge loss would be 60 db.

Obviously many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

I claim:

particular slow wave structure may be of the same 1. In a traveling wave tube having means for providing a stream of electrons along a predetermined path and means for propagating electromagnetic wave energy comprising means defining a. plurality of intercoupled interaction cavities in a manner so that an interaction takes place with the stream of electrons, the improvement comprising:

means for preventing band edge oscillations includ ing a plurality of irises coupled to the auxiliary cavities disposed proximate and externally to said predetermined path such that the longitudinal axis of said means for preventing the band edge oscillations is parallel to said predetermined path means for cooling the auxiliary cavities; and

means for isolating the cooling means from the auxiliary cavities.

2. The device as claimed in claim 1 wherein said cooling means includes an elongated, electrically nonconductive tube adapted to receive fluid.

3. The device as claimed in claim 2 wherein said oscillation preventing means includes an elongated, electrically conductive tube disposed coaxially within said electrically non-conductive tube. V

4. The device as claimed in claim 3 wherein the outer diameter of the electrically conductive tube is equal to the inner diameter of the electrically non-conductive whereby the tubes are in contact with each other.

5. The device as claimed in claim 3 wherein the outer diameter of the electrically conductive tube is smaller than the inner diameter of the electrically nonconductive tube whereby fluid can flow between the tubes.

6. The device as claimed in claim 2 wherein said oscillation preventing means includes an elongated, high loss tube disposed coaxially within said electrically non-conductive tube.

7. The device as claimed in claim 6 wherein said oscillation preventing means further includes anelectrically conductive rod disposed coaxially within said high loss tube.

8. The device as claimed in claim 2 wherein:

said non-conductive tube is comprised of a low loss material; and

the fluid is a lossy fluid. 

1. In a traveling wave tube having means for providing a stream of electrons along a predetermined path and means for propagating electromagnetic wave energy comprising means defining a plurality of intercoupled interaction cavities in a manner so that an interaction takes place with the stream of electrons, the improvement comprising: means for preventing band edge oscillations including a plurality of irises coupled to the auxiliary cavities disposed proximate and externally to said predetermined path such that the longitudinal axis of said means for preventing the band edge oscillations is parallel to said predetermined path means for cooling the auxiliary cavities; and means for isolating the cooling means from the auxiliary cavities.
 2. The device as claimed in claim 1 wherein said cooling means includes an elongated, electrically non-conductive tube adapted to receive fluid.
 3. The device as claimed in claim 2 wherein said oscillation preventing means includes an elongated, electrically conductive tube disposed coaxially within said electrically non-conductive tube.
 4. The device as claimed in claim 3 wherein the outer diameter of the electrically conductive tube is equal to the inner diameter of the electrically non-conductive whereby the tubes are in contact with each other.
 5. The device as claimed in claim 3 wherein the outer diameter of the electrically conductive tube is smaller than the inner diameter of the electrically non-conductive tube whereby fluid can flow between the tubes.
 6. The device as claimed in claim 2 wherein said oscillation preventing means includes an elongated, high loss tube disposed coaxially within said electrically non-conductive tube.
 7. The device as claimed in claim 6 wherein said oscillation preventing means further includes an electrically conductive rod disposed coaxially within said high loss tube.
 8. The device as claimed in claim 2 wherein: said non-conductive tube is comprised of a low loss material; and the fluid is a lossy fluid. 